The term “digital positioner” used in this disclosure stands for a mechatronic system, which controls the auxiliary power of a pneumatic actuator according to one or more input signals in order to bring the final control element into a particular position. The digital pneumatic positioner requires pressurized gas as the auxiliary power and electrical power in order to work.
The generic digital pneumatic positioner comprises at least the main components described in greater detail below. With a pneumatic system, the flow of gas to or from the chambers of a single-acting or double-acting pneumatic drive is controlled systematically as a function of one or more input signals. Using position feedback sensors, the movements and positions of the final control element are represented as one or more signals. In addition, there is an electronic control unit, which comprises a microcontroller and receives one or more input signals. The firmware in the electronic control unit processes the input signals and the signals from the position feedback sensors into output signals that are used as input signals to the pneumatic system.
The firmware of the digital positioner implements a function that analyses the dynamic properties of the connected actuator. At commissioning, the final control element is moved through its operating range once during an initialization process, and the initial value and final value of the operating range is recorded.
Actuators are classified as rotary or linear drives. In the linear drive, the linear movement of the drive of the actuator is transferred directly to a linearly actuated final control element. In contrast, in the rotary drive, the linear movement of the drive of the actuator is converted into a rotation by suitable means.
The actuator and the digital positioner are connected by a mounting set. The mounting set comprises components that transfer the movement and position of the actuator to the position feedback sensors of the positioner.
The known measuring systems of the position feedback sensors measure either linearly or rotationally, as known from DE 42 39 635 A1. If the effective direction of the actuator differs from the effective direction of the measuring system, the measurement must be linearized in order to ensure that positioning is performed sufficiently accurately. It is essential for this, however, to know in advance whether the given drive type is rotary or linear. For the known positioner, the operating person must provide this information by a manual setting during commissioning, which is prone to errors.